1 Jim Thomas - LBL A proposal to close the Grid Leak “it will never die” Yuri Fysiak, Jim Thomas & Gene VanBuren Eric Anderssen, Howard Wieman & Jon Wirth 9/26/2015

2 Jim Thomas - LBL Proposal 1 to close the “Grid Leak” Wall is 1.8 mm thick Printed Circuit Board with Cu on both sides Grey bumps are 1 M Ohm type 603 surface mount resistors -115 Volts matches the GG voltage when it is “open” -230 Volts provided by external power supply, -115 V provided by resistors to 0 Volts

3 Jim Thomas - LBL Yuri’s Simulation Geometry Pad Plane on top, Central Membrane at bottom Rotate ME views 180  around an axis coming out of the page Electrons float up, Ions fall down Note that Central Membrane is only 5 mm from the GG Outer Pad Plane Inner Pad Plane Fat Anode Wire 20  m Anode wires Gated Grid wires Ground Grid wires Central Membrane

4 Jim Thomas - LBL Normal Geometry: No Wall Geometry Equipotential Lines Ion Flux 1 e - /cm in, ~10 ions/cm out Grid Leak

5 Jim Thomas - LBL -230 volts on outside of wall

6 Jim Thomas - LBL -690 Volts on outside of wall

7 Jim Thomas - LBL Proposal 1: Conclusions A Biased wall will work Order of magnitude decrease of Ion Flux can be achieved ~700 volts required to do a good job Note that equipotential lines (lower right panels) guide the ions to the wall Mechanical Engineering is achievable –Requires 0.070” deep notch in top and bottom of inner sector so 1/16” G10 wall can be glued or screwed in place –Requires modest change in routing of Pad Plane, perhaps want to redesign ground plane on Pad Plane to reflect new routing pattern –One bias connector available on inner sector, have to run cable down existing groove on side (under pulser and GG boards) –Need to route bias cable around the corner of the sector to access wall –Want to do this on both top and bottom of Inner Sector (grid leak in both places)

8 Jim Thomas - LBL Proposal 2 to close the “Grid Leak” Volts -115 Volts 0.0 Volts Volts Volts 2.0

9 Jim Thomas - LBL Grounded fingers between the grids: Inner & Outer Sectors!

10 Jim Thomas - LBL Proposal 2: Conclusions Grounded Fingers will work – very simple conceptual design Order of magnitude decrease of Ion Flux can be achieved However, this solution requires changes to the outer sectors while still installed inside the TPC –Hands, heads, and tools inside the TPC! Epoxy, too. –Careful fixturing may make this solution possible –Engineering required, Risk analysis required Mechanical Engineering is achievable –Requires 0.070” deep notch in top and bottom of inner sector so 1/16” G10 wall can be glued or screwed in place –Requires modest change in routing of Pad Plane, perhaps want to redesign ground plane on Pad Plane to reflect new routing pattern A careful analysis of the equipotential lines (lower right panel) suggests that a river of ions flows very close to the GG and out the gap –The grid leak will never completely die … the river of ions will exist as long as there is a physical gap between the inner and outer gated grids

11 Jim Thomas - LBL Grand Conclusion Further work is needed before we can decide upon which solution is best: Biased Wall or Fingers between the grids –Is it reasonable to put ~700 volts on a wall near the wires? –Engineering design & Fixturing required for “Finger” solution –Risk analysis required for both proposals Both designs can/should be further optimized –But is seems unlikely that we can kill the grid leak substantially more than a factor of 10 below the current design The Mechanical Engineering design for the Strongback is the same for both of these solutions –We should add a 0.070” notch on the top and bottom sides of the inner sector to accommodate a 1/16” G10 (or Cu) Wall –Assume that we will run a cable to bias the wall, but we may not use it I believe the Mechanical Engineering design of the strongback is complete. We can now proceed with final drawings and fabrication of prototypes (JT).

12 Jim Thomas - LBL Backup Slides

13 Jim Thomas - LBL Location of Wires and Pads Radius (Y) Description 0.00Center of STAR Detector (vtx) Bottom of Full size PC Board Tertiary Fiducial L & R Strongback Bottom Edge Gated Grid Wire Gated Grid Wire Anode Wire 1 & GG W Anode Wire 2 & GG W Anode Wire 3 & GG W Secondary Fiducial Anode Wire 4 & GG W Anode Wire 5 & GG W Pad Row 1 - Center Pad Row 2 - Center Pad Row 39 - Center Primary Fiducial Pad Row 40 - Center Anode Wire 166 & GG W Anode Wire 167 & GG W Anode Wire 168 & GG W Anode Wire 169 & GG W Alternate Primary Fiducial Anode Wire 170 & GG W Gated Grid Wire Gated Grid Wire Strongback Top Edge Tertiary Fiducial L & R Top of Full size PC Board Repeat pad rows every 16 mm Wire Locations are the same as before except for the replacement of 6 thin anode wires with larger diameter anode wires (0.020 mm  mm) References: LBL Drawings 24A055, 24A373, 24A374